The older Mercedes diesels hold up to time well, but they must be maintained. I should warn you not to buy one without driving it and inspecting it closely, as there can be many serious problems that are not immediately obvious. The engine may start and run well when warm, but it can still be worn out and not start well when cold. A compression test is a good test for this. Make sure the car steers and handles well -- lots of these vehicles have traveled many many miles and the rubber in the suspension breaks down. Case in point, my own "cheap" Mercedes was driven 300k miles, only the last 50k by me. This winter I had a sway bar end link break, and the reason was water seeped in past the old hard rubber bushings and rusted out the end of the bar. The result was a dangerous pull to the left only when the brakes were applied. The repair was cheap, but many of these may lurk. Be prepared to work on the car you end up with, regardless of how much you end up paying.
This comment has been crossposted at AT&T: 611 Folsom Street, San Francisco, CA -- Room 641A.
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'05 Liberty
'83 240D